Saint Anthony Falls was originally called Saint Anthony's Fall, named by Father Louis Hennepin who brought the Falls to the attention of the world. Hennepin County where the falls are located bears his name. He named the falls them after his patron saint.
Saint Anthony's Falls and Climax Prairie (the destination) are listed in the book "Post Office Directory, or Business Man's Guide to the Post Offices in the United States" by D.D.T Leech, published in 1866. The cancel date is most likely 11/21/1865.